Owing to the small differences in the relative volatilities of the components of the C4 cut, obtaining 1,3-butadiene from a C4 cut is a complicated distillation problem. Therefore, the separation is carried out by extractive distillation, i.e. a distillation with addition of an extractant which has a higher boiling point than the mixture to be separated and which increases the differences in the relative volatilities of the components to be separated. The use of suitable extractants allows a crude 1,3-butadiene fraction to be obtained from the C4 cut mentioned by means of extractive distillation, and said fraction is subsequently further purified in purifying distillation columns.
In the present context, crude 1,3-butadiene refers to a hydrocarbon mixture which has been obtained from a C4 cut from which at least 90% by weight of the sum of butanes and butenes, preferably at least 98% by weight of the sum of butanes and butenes, more preferably at least 99% by weight of the sum of butanes and butenes, and simultaneously at least 90% by weight of the C4 acetylenes, preferably at least 96% by weight of the C4 acetylenes, more preferably at least 99% by weight of the C4 acetylenes, has been removed. Crude 1,3-butadiene contains the 1,3-butadiene product of value frequently in a proportion of at least 80% by weight, preferably 90% by weight, more preferably more than 95% by weight, remainder impurities.
Accordingly, pure 1,3-butadiene refers to a hydrocarbon mixture which contains the 1,3-butadiene product of value in a proportion of at least 98% by weight, preferably of at least 99.5% by weight, more preferably in the range between 99.7 and 99.9% by weight, remainder impurities.
DE-A 101 05 660 discloses a process having simplified constructional design of the apparatus compared to prior processes: the C4 cut is separated in a dividing wall column having a dividing wall extending up to the upper end of the dividing wall column and an extractive wash column upstream of the dividing wall column. According to the process of DE-A 101 05 660, a semidegassed solvent stream is drawn off from the bottom of the dividing wall column used for the extractive distillation. The term “semi-degassed solvent” is familiar to those skilled in the art who work in extractive distillation to obtain 1,3-butadiene and refers to a selective solvent which still contains dissolved components from the C4 cut to be separated, specifically the components which have the greatest affinity for the selective solvent. These include in particular the C4 acetylenes, in particular ethylacetylene and vinylacetylene.
However, a solvent stream which has merely been semidegassed cannot be recycled into the extractive distillation, since the acetylenes damaging to the specification would otherwise accumulate. It was therefore necessary to feed the bottom stream drawn off from the dividing wall column, before the recycling into the extractive distillation, initially to an outgasser column, as disclosed, for example, by DE-A 27 24 365, which is operated at lower pressure compared to the column from whose bottom the semidegassed stream is drawn off. In the outgasser column, the semidegassed solvent stream is processed to obtain a purified, i.e. fully degassed, solvent at the bottom and a gaseous hydrocarbon stream at the top of the outgasser column, which is recycled via a compressor into the lower region of the extractive distillation column. The acetylenes are discharged via a sidestream.
However, according to the process of DE-A 27 24 365, the bottom stream which is drawn off from the dividing wall column and is fed to the outgasser column contains, in addition to the C4 acetylenes, also considerable amounts of the 1,3-butadiene product of value. The 1,3-butadiene goes into the top stream of the outgasser column, which, in an economic operating mode, cannot be discarded, but rather is recycled via a compressor into extractive distillation which is operated at higher pressure compared to the outgasser. The compressor has high energy consumption; the process of DE-A 27 24 365 was therefore actually an advance over prior processes, in which compressors having triple the energy consumption were required. However, at the application date of DE-A 27 24 365, it was unknown to those skilled in the art that a process version which can fully dispense with the compressor can be realized in a technically simple manner.
DE-A 103 22 655 describes a process in which the control of the energy input into the dividing wall column via the bottom evaporator thereof and the configuration of the number of theoretical plates in the lower combined column region can be used to adjust the operation of the dividing wall column in such a way that a bottom stream can be removed from the dividing wall column and already contains purified solvent.
Accordingly, both the outgasser column and the compressor for recycling 1,3-butadiene-containing stream into the extractive distillation become unnecessary.
In the present context, the term purified solvent or fully degassed solvent refers to a solvent which has been depleted in components from the C4 cut to such an extent that it is suitable for use as a selective solvent for the extractive distillation of a C4 cut, while the predefined specifications for crude 1,3-butadiene and raffinate 1 are complied with. Key components in this context are C4 acetylenes, especially ethylacetylene and vinylacetylene.